As well known, bonded magnets are lighter in weight and higher in dimensional accuracy than sintered magnets and suitable for mass production of articles having complicated shapes, and therefore they are widely used for various magnet applications. Recently, magnet-applied products have been drastically miniaturized and reduced in weight, requiring high-performance ferrite bonded magnets having a higher Br (or higher Br and iHc) suitable for miniaturization and reduction in weight.
Conventional Sr and/or Ba ferrite bonded magnets are obtained by bonding Sr and/or Ba ferrite powder having a composition represented by AO.nFe.sub.2 O.sub.3, wherein A is Sr and/or Ba, and n=5-6, with binders such as thermoplastic polyolefin resins or rubbers, advantageous in low cost. However, the ferrite bonded magnets are lower in Br and maximum energy product (BH).sub.max than sintered ferrite magnets to the extent of volume increase due to non-magnetic portions occupied by binders. To obviate this disadvantage, various attempts have been made conventionally to improve the orientation of a ferrite powder by a magnetic field or a mechanical stress applied for a ferrite powder orientation, and to improve the filling of a ferrite powder in binders. As a result, it is almost considered that no further improvement in magnetic properties would be able to be achieved in bonded magnets comprising a ferrite powders having conventional compositions.
If the filling ratio of a ferrite powder in rubbers or plastics is increased to improve the magnetic properties of bonded magnets, the resultant blends would have extremely high melt viscosity. Even though high-melt viscosity blends are subjected to practical orientating magnetic field or mechanical stress, it would be difficult to obtain bonded magnets having well-oriented ferrite powder. This difficulty is remarkable in an injection molding method, though it is appreciable in an extrusion method and a compression molding method, too. Though the orientation of a ferrite powder in the ferrite bonded magnets is improved by increasing the filling ratio of a ferrite powder in rubbers or plastics, such improvement inevitably causes the deterioration of magnetic properties, failing to satisfy the demand of miniaturization and reduction in weight.
To obviate such problems of conventional technology, it is effective to improve the saturation magnetization .sigma.s or crystal magnetic anisotropy constant of a ferrite powder for bonded magnets. Improvement in .sigma.s directly leads to improvement in coercivity Hc (or iHc). Incidentally, the conventional a ferrite powder for bonded magnets having a composition of AO.nFe.sub.2 O.sub.3 has a magnetoplumbite-type crystal structure, and W-type ferrite having larger .sigma.s than a ferrite powder having a magnetoplumbite-type crystal structure has also been investigated. However, the mass production of the W-type ferrite cannot be materialized so far due to difficulty in the control of a sintering atmosphere.
Japanese Patent Laid-Open No. 9-115715 discloses a ferrite powder for bonded magnets having a main phase constituted by a hexagonal magnetoplumbite-type ferrite represented by the general formula: A.sub.1-x R.sub.x (Fe.sub.12-y M.sub.y).sub.z O.sub.19, wherein A is at least one element selected from the group consisting of Sr, Ba, Ca and Rb, R is at least one of rare earth elements including Y, La being indispensable, M is Zn and/or Cd, and x, y and z are molar ratios meeting the conditions of 0.04.ltoreq.x.ltoreq.0.45, 0.04.ltoreq.y.ltoreq.0.45, and 0.7.ltoreq.z.ltoreq.1.2. Investigation by the inventors has revealed, however, that it is difficult to obtain bonded magnets having high Br and iHc (for instance, exceeding 3.5 kOe) from this a ferrite powder for bonded magnets.
Accordingly, an object of the present invention is to provide a high-performance bonded magnet having a magnetoplumbite-type crystal structure suitable for mass production, which has higher Br (or higher Br and iHc) than those of conventional Sr and/or Ba ferrite bonded magnets, a magnet roll composed of such a bonded magnet, a ferrite powder used for such a bonded magnet, and methods for producing them.